In today's telecommunication network, optical fibers are typically chosen over electrical cablings to transmit information in the form of light from one place to another partially because of various advantageous characteristics of the optical fibers, for example, a higher bandwidth, a longer transmission distance, etc., when compared to the electrical cablings.
To further increase the bandwidth of the optical fiber, multiplexing a plurality of optical signals on one optical fiber by using respective different wavelengths of light has been proposed, for example, a dense wavelength division multiplexing (DWDM) technique. In general, a photonic device (e.g., modulator), coupled to the optical fiber, is typically used to differentiate (e.g., divide) such a plurality of optical signals by using optical gratings to diffract the plurality of optical signals. For example, a photonic device may include a plurality of optical gratings, each of which is formed as a comb-like structure extending into a substrate with a respective depth. When the photonic device receives a plurality of optical signals that are associated with respective different wavelengths, based on the different depths, each optical grating can let one optical signal of a corresponding wavelength to pass through.
Existing techniques to make photonic devices having such a plurality of comb-like structures with respective different depths typically rely on using one single mask layer to directly etch the substrate multiple times. Such techniques, however, can cause various issues such as, for example, undesirable residues (e.g., reacted photoresist materials) remained in the formed comb-like structures, which disadvantageously impacts performance of the photonic devices. Thus, existing photonic devices and methods to make the same are not entirely satisfactory.